There are already known for quite some time processes suited for the preparation of synthetic polymeric fibers endowed with such a morphology as to be able to replace wholly or in part the cellulose fibers in the preparation of paper and other such like materials.
Particularly suited synthetic fibers, known in the technical field as "fibrils" or "plexofilament fibrids", have as a general characteristic a high surface area, in general of at least 1 sq.mt/gram, a length comprised between about 0.5 and 10 mm, and a diameter comprised between 1 and 100 microns.
Processes for their preparation are described, for instance, in: British Pat. Nos. 868,651, 891,945, 1,262,531, as well as in German patent application No. 2,343,543, in Belgian Pat. No. 789,808, in U.S. Pat. Nos. 3,770,856, 3,750,383 and 3,808,091 and, finally, in Italian Pat. No. 947,919.
However, particular difficulties are met with in the use of the polyolefinic fibers especially in the field of paper-making, mainly due to the non-polar nature and low density of the polymers, wherefore they are not at all wettable and dispersible in water, while at the same time they display a very poor self-cohesion in the context of leaf-like or panel-like structures, which thus have a breaking length of less than 100 mt.
In the prior art there are known various methods for improving the water dispersibility and the self adhesion power of the polyolefine fibers used in the field of paper-making.
According to French Pat. No. 2,153,941 said fibers are subjected to surface reaction with hydrophile groups of the --SO.sub.3 H and --COOH type.
According to another method, disclosed in French patent application Nos. 2,223,442 and 2,257,635, polyolefine fibers are treated with acid aldehydes-modified polyvinyl alcohol, or acetalized polyvinylalcohol, respectively.
At last, in Belgian Pat. No. 787,060 there is disclosed a process which consists in heating a mass of polyolefine fibers, imbued with and/or swollen by a solvent, into an aqueous solution of polyvinylalcohol, up to elimination of the solvent.
According to all these methods only advantages concerning the dispersibility in water are obtained, whereas the self-adhesion power of the fiber is only scarcely improved.
Furthermore, if one tries to develop such characteristics by subjecting the thus obtained synthetic pulp to a refining operation, one obtains only a partial dissolving of the fiber bunches, with a remarkable worsening of the toughness, cohesion and freeness value of the fibrous material, besides an increase in the number of the clots.